Abstract
Four types of gravitational lenses are investigated here. Their common feature is "marginality": one type has a pair of merging images, another is a lens barely able to split images, the third is a lens made of a galaxy superposed on a cluster at a site where merging images produced by the cluster alone could have been, the fourth is made of a galaxy superposed on a cluster almost able to split images on its own. The marginal gravitational lenses are important if either or both of the luminosity and density bias hypotheses are true. The first hypothesis assumes a very steep quasar luminosity function, the second assumes a very steep falloff of the distribution of clusters in surface mass densities. A crude model statistics, based on available statistics of clusters and quasars gives an estimate for the uncertainties in numbers and characteristics of lenses of large separations, on the basis of the presently available data.
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